1. Field of the Invention
The invention relates generally to electronic circuits and, more particularly, to circuits for wireless communication.
2. Description of the Related Art
Conventional radios have been extensively implemented using superhetrodyne and direct conversion architectures constructed from circuit blocks such as mixers, amplifiers, and RF filters. There are many chipsets on the market using various process technologies, but they are usually active circuits requiring a power source. One alternate approach is to construct a radio using a passive surface acoustic wave (SAW) expander and correlator.
FIGS. 1 and 2 show a representative transmitter 10 and receiver 20, respectively, using the SAW approach. The main disadvantage of SAW correlator and expander implementations in the prior art is that they are implemented at an intermediate frequency (IF) to allow a local oscillator (LO) (12, 22) to compensate for inherent temperature drift and tolerances of the SAW device (11, 21) as a closed loop system. Although the SAW device (11, 21) itself is passive in this implementation, other active circuits are still required at the front end and for the LO and up/down converters, as shown. Without such compensation, the correlation performance and sidelobe levels become unacceptable over a practical temperature range, making the radio performance unacceptable. Historically, correlators/expanders have not been implemented directly at the radio frequency (RF) front end because of the temperature drift problem described above, but also because of problems with device to device manufacturing variance and difficulties with small feature size lithography required for operation above about 800 MHz.
Thus, it would be desirable to have a correlator/expander implemented at the RF front end if the issues of temperature variation and tight lithographic technology could be resolved.